1. Field of the Invention
The present invention relates to a trochoid gear type fuel pump constituted by eccentrically arranging an inner gear at an inner peripheral side of an outer gear.
2. Description of the Related Art
In recent years, for the purpose of improving fuel discharge performance of a fuel pump mounted in a vehicle, it has been considered to adopt a trochoid gear type fuel pump. As shown in FIG. 7, the trochoid gear type fuel pump is constructed such that an inner gear 3 having outer teeth is eccentrically arranged at an inner peripheral side of an outer gear 2 having inner teeth which is rotatably housed in a cylindrical pump casing 1, both the gears 2, 3 are engaged with each other to form pump chambers 4 between the teeth of both the gears 2, 3, and a driving motor (not shown) drives and rotates the inner gear 3 to rotate the outer gear 2, so that while the pump chambers 4 between the teeth of both the gears 2, 3 are moved in a rotation direction, the volumes of the pump chambers 4 are continuously increased and decreased to suck and discharge fuel.
Since this sort of trochoid gear type fuel pump repeats a volume change of the pump chamber 4, a discharge pressure pulsation of a frequency corresponding to the number of teeth of the inner gear 3 is generated, and the discharge pressure pulsation vibrates a fuel tank, fuel piping, a floor panel of a vehicle, and the like, so that there is a problem that noise and vibration becomes large. On this account, in the case where the trochoid gear type fuel pump is used, for the purpose of reducing the noise and vibration, it is necessary to take measures against the noise, for example, a discharge pressure pulsation reducing device is attached to the outside of the fuel pump, or a sound shielding member is bonded to a vehicle body, and therefore, there is a defect that costs are increased.
In the trochoid gear type fuel pump, after fuel is sucked into the pump chamber 4 in a region where the volume of the pump chamber 4 is increased by the rotation of both the gears 2, 3, the fuel in the pump chamber 4 is pressurized and discharged in a region where the volume of the pump chamber 4 is decreased. Here, in the discharge region where the volume of the pump chamber 4 is decreased, the fuel in the pump chamber 4 is pressurized and the pressure of the fuel (fuel pressure) is raised, so that a load in an outer diameter direction is applied to the outer gear 2 by the rise of the fuel pressure. Since such load in the outer diameter direction by the rise of the fuel pressure is not generated in the suction region (suction port side) where the fuel pressure in the pump chamber 4 is lowered, the load in the outer diameter direction to the outer gear 2 affects only the discharge region (discharge port side) where the fuel pressure of the pump chamber 4 is raised, and this becomes an eccentric load to cause a state where a part of the outer gear 2 at the discharge port side is strongly pressed to the inner peripheral surface of the pump casing 1. Thus, sliding resistance (friction loss) of the outer gear 2 to the pump casing 1 becomes large, and the load of the driving motor becomes high by that, so that there are such defects that consumed electric power is increased, and the lowering of the fuel discharge performance and lowering of pump rotation speed are caused.
Further, in FIG. 7, since it is necessary to provide a clearance between the outer periphery of the outer gear 2 and the inner periphery of the pump casing 1 in view of production tolerance, sliding resistance, and the like, there has been a defect that jolting and whirling are produced in the clearance, and by that, the outer gear 2 collides against the inner peripheral surface of the pump casing 1, and noise and vibration become large.
In JP-A-5-133347, a clearance between an outer periphery of an outer gear and an inner periphery of a pump casing is made large, and the outer periphery of the outer gear is elastically supported by an elastic support mechanism at 120xc2x0 intervals, and when a foreign matter intrudes into the clearance between the outer periphery of the outer gear and the inner periphery of the pump casing, the outer gear moves in the direction opposite to the intruding position of the foreign matter, so that a lock of the outer gear by engagement of the foreign matter is prevented. However, as in this publication, when such structure is adopted that the clearance between the outer gear and the pump casing is made large, and the outer gear is raised in regard to the pump casing by the elastic support mechanism and is elastically supported, it becomes more difficult to reduce the whirling of the outer gear than the prior art, and the whirling of the outer gear is amplified by contraries, so that an adverse effect is produced on the noise and vibration, and results in the increase of noise and vibration.
The present invention has been made in view of these circumstances, and a first object thereof is to provide a fuel pump which can reduce noise and vibration due to a discharge pressure pulsation at low cost. A second object thereof is to provide a fuel pump which reduces sliding resistance (friction loss) of an outer gear to a pump casing and can realize a reduction in consumed electric power and an improvement in fuel discharge performance of a driving motor.
In order to achieve the first object, a trochoid gear type fuel pump according to a first aspect of the present invention is structured such that two pumps made of an outer gear and an inner gear are provided, and phases of discharge pressure pulsations of the two pumps are shifted from each other by an almost half wavelength (half period) and are merged while interfering with each other. By doing so, when a pressure pulsation wave of fuel discharged from the one pump has a peak, the other has a bottom, and the discharge pressure pulsations of the two pumps interfere with each other to attenuate, so that the discharge pressure pulsation of the fuel pump is greatly reduced, and the noise and vibration due to the discharge pressure pulsation is greatly reduced. By this, the conventional noise measures (discharge pressure pulsation reducing device, sound shielding member, etc.) become unnecessary, and low noise and low vibration can be realized at low cost.
In this case, as a structure where the phases of the discharge pressure pulsations of the two pumps are shifted from each other by an almost half wavelength and are merged, the following two structures are conceivable. For example, if such a structure is adopted that lengths of fuel flow paths from discharge ports of two pumps to a fuel confluent portion are shifted from each other by an almost half wavelength (or odd number times as long as the half wavelength), the phases of the two discharge pressure pulsations are shifted from each other by the almost half wavelength at the fuel confluent portion, and the discharge pressure pulsations interfere with each other to attenuate.
Further, such a structure may be adopted that outer gears of two pumps are integrally formed, two inner gears are eccentrically arranged at an inner peripheral side of one outer gear in a state where they are overlapped with each other through a partition wall, and eccentric directions of both the inner gears with respect to the outer gear are shifted from each other by 180xc2x0 to the opposite side. According to this structure, in the two inner gears arranged at the inner peripheral side of the outer gear, since the eccentric directions of both are shifted from each other by 180xc2x0 to the opposite side, fuel pressure rising sides (discharge port) in the two inner gears are shifted from each other by 180xc2x0 to the opposite side. By this, since loads in the outer diameter direction by the rise of fuel pressure affect the one outer gear from the two inner gears oppositely to each other by 180xc2x0, the loads in the outer diameter direction affecting the outer gear are balanced, and an eccentric load hardly affects the outer gear. Thus, there does not occur such a state where the outer gear is strongly pressed to the inner peripheral surface of the pump casing by the fuel pressure, and the sliding resistance (friction loss) of the outer gear to the pump casing becomes lower than the prior art, and by that, the load of the motor is decreased, and the consumed electric power is decreased. Further, since fuel is sucked and discharged by the two inner gears in the outer gear, in cooperation with the foregoing sliding resistance reduction effect, fuel discharge performance can be effectively raised. By this, this structure can achieve both the first and second objects.
Further, such a structure may be adopted that discharge ports through which fuel in a pump chamber is discharged are formed at two places, and phases of discharge pressure pulsations of the discharge ports at the two places are shifted by an almost half wavelength and are merged while interfering with each other. By doing so, the discharge pressure pulsations of the two discharge ports interfere with each other to attenuate, the discharge pressure pulsation is greatly reduced, and the noise and vibration due to the pressure pulsation is greatly reduced. By this, as compared with the case where two pumps are provided, the number of parts can be decreased and the structure can be simplified, and miniaturization, reduction in weight, and reduction in cost can be realized.
Further, a third object of the present invention is to provide a trochoid gear type fuel pump which can reduce noise and vibration due to jolting and whirling.
In order to achieve the above object, according to an aspect of the present invention, a trochoid gear type fuel pump is provided with elastic press means for pressing an outer gear to a cylindrical pump casing in one direction by an elastic force. When the outer gear is pressed to the pump casing in one direction, since the outer gear rotates in a state where it is pressed to a constant position of an inner peripheral surface of the pump casing, jolting and whirling of the outer gear can be suppressed, and noise and vibration due to the jolting and whirling can be effectively reduced.